Normal sera contains an IgG antibody which normally exists in an inactive complexed state with IgG antiglobulin. Once disassociated this antibody is moderately cytotoxic to tumor cells. Cross reactivity was demonstrated by adsorption with tumor cell membrane extracts between many but not all tumors. This was not restricted to an individual tumor class. The antigen(s) may be of fetal origin since cross reacting antigen was also found on fetal brain and stomach cells. Melanoma sera with low levels of complexes was directly cytotoxic while similar sera with high levels of complexes required uncoupling of the complexes for cytotoxicity to occur. The cytotoxin in melanoma serum complexes has IgM specificity while the complexes in normal serum contains a cytotoxin of IgG specificity.